Download - The Physics of Ultrasound Eei
Extended Response Task
2
Year 12 Physics
Miss Meni
Semester 4, Term 3
Due Date: 19th August
2011
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Table of Contents
1.0 Introduction:.......................................................................3
2.0 History of the technology:....................................................3
3.0 Analysis of technology design and function:..........................6
3.1 Wave Propagation:................................................................................7
3.2 Piezoelectric Transducer:......................................................................8
3.2.1 Electrostriction................................................................................9
3.2.2 The Piezoelectric Effect.................................................................11
3.3 Attenuation:........................................................................................11
3.4 Acoustic Impedance:...........................................................................13
4.0 Comparison to alternatives:................................................15
5.0 Future developments:........................................................17
6.0 Conclusion:........................................................................19
7.0 References:.......................................................................20
8.0 Appendices:.......................................................................22
8.1 Appendix 1:.........................................................................................22
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The Physics of Ultrasound
1.0 Introduction:
Medical imaging is the technique used by doctors to create internal
images of the human body for clinical reasons such as examining and
diagnosing diseases (NDT Resource Centre, 2011). The aim of medical
imaging is to deliver an image of inside the body in a way that is as non-
invasive as possible (Radiological Society of North America, 2011).
This report will investigate ultrasound as a form of medical imaging and
determine the history of the technology, comparison to alternative forms
of imaging, future developments and will investigate the physics behind
ultrasound in all regards.
2.0 History of the technology:
Prior to modern day ultrasonic testing, numerous physics concepts had to
be first discovered. The following is a timeline of physics discoveries that
lead to the common use of ultrasound imaging (see figure 1).
500BC
The greek philosopher and mathematician Pythagoras experimented with the vibration of strings.This is the earliest study of sound and was the beginning of the idea that sound travels in waves.
400BC
Another greek philospher, Aristotle, suggested that the movement of air carries sound.This again contributed to to the modern day use of ultrasound as this theory explains the movement of sound waves.
150AD
Leonardo DaVinci discovered and proved that sound travels in waves.This proved Aristotles theory and again contributed to the idea of ultrasound.
Figure 1: Timeline of Ultrasound
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1640
Marin Merenne first measured the speed of sound through air.
1660
Robert Boyle discovered that that sound waves require a medium to propagate.
1665-
1669
Sir Isaac Newton formulated a relationship between the speed of sound in a medium and the density and compressiblity of a medium.
Mid 1700's
Daniel Bernoulli explained that a string can vibrate at more than one frequency.
1790
Lazzaro Spallanzani experimented with bats and discovered that they flew through the air using hearing rather than sight.This lead to the theory that sounds waves can be reflected or echoed back to the original source. This is still the basis of all ultrasound testing.
1826
The swiss physicist/engineer Jean-daniel Colladon discovered sonography with an underwater bell. With this, he accurately determined the speed of sound through water.
1881
Pierre Curie found a connection between between electrical voltage and pressure on a crystalline material, I.e. The piezoelectric effect.This was the biggest breakthough in the history of ultrasound since the discovery of sound waves as it is the basis behind the modern day piezoelectric transducer.
1912-
1915
After the sinking of the titanic, Paul Langevin invented the first hydrophone to detect icebergs. This was the first transducer; a device that could send and receive low frequency sound waves. This was another major step foward in the history of ultrasound.
Late
1930's
The Austrian psychiatrist Dr. Karl Dussik was the first to use ultrasound pictures in an attempt to diagnose brain tumors.The procedure was called "hyperphonography" and he used heat sensitive paper to record the echos.This is considered the first the first use of ultrasound for medical purposes.
Continued next
page.
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Late 1940's
Dr. George Luwig was the first to record the difference in sound waves as they travelled through tissues, organs, muscles and gallstones.This once again was an extravigant contribution to the history of ultrasound.
1950's
Scottish professor Ian Donald invented the B mode scanner.
1960's
Douglas Howry and Joseph Holmes impoved the B-mode scanner by inventing a tranducer that came into direct contact with the patient. Before this, patients had to be submerged in water.This was the start of ultrasound as we know it today.
1970 -
present
Dr. John Wild and John Reid improved various ultrasound equipment such as the B-mode instrument that could swing side to side and image patients from different angles. This instrument was used to to detect breast tumors.They also invented an A-mode scanner for the detction of ovarian cancer.
(Genesis Ultrasound, 2010. Simmons, n.d).
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3.0 Analysis of technology design and function:
Ultrasound is form of mechanical sound energy that travels through the
body in order to produce an internal image (Ultrasound for Regional
Anesthesia, 2008). Ultrasound is used in medicine for the purpose of
examining and diagnosing a variety of issues within the human body. This
form of medical imaging helps physicians to assess symptoms such as
pain, swelling and infections in many of the body’s internal organs such as
the heart, liver, gallbladder, spleen, pancreas, brain, kidneys and most
commonly it is used for examining an unborn child (fetus) in pregnant
patients (Radiological Society of North America, 2011).
Several functional units are required for a typical ultrasound inspection
system. These functional units include: a pulsar/receiver, a transducer and
display services. The pulsar/receiver is an electronic device that produces
high voltage electrical pulses. Driven by the pulsar, the transducer
generates high frequency ultrasonic energy, which in turn produces sound
energy that propagates through materials in the form of longitudinal
sound waves (NDT Resource Centre, 2011). When a defect (e.g. A crack) is
in the wave path, part of the energy is reflected back to the transducer
from the surface of the flaw (see figure 2). The transducer then converts
the reflected wave signal back to an electrical signal and the result is
displayed on a screen (NDT Resource Centre, 2011).
Figure 2: Generation of Ultrasound
(NDT Resource Centre, 2011)
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3.1 Wave Propagation:
Sound travels as a longitudinal, mechanical wave. Sound waves are
capable of traveling through any medium (e.g. Body tissue), however
without a medium, sound does not exist. The medium itself is in fact
motionless, however, the individual atoms and molecules in the medium
oscillate about their equilibrium position and interact with the
neighbouring molecules, thus transferring energy (The Physics Classroom,
n.d). These waves exist as a disparity of pressure. An area of increased
pressure on a sound wave is called a compression, whereas a region of
decreased pressure on a sound wave is called a rarefaction (Media
College, n.d)(see figure 3).
The speed of sound is the rate of the transmission of energy along a chain
of particles. It depends solely on the medium it is traveling through.
Predominantly, the elastic and inertial properties have the greatest affect
on the velocity of the wave (The Physics Classroom, n.d). With relation to
ultrasound, the speed of sound fluctuates for different biological mediums;
however, the accepted average for the speed of sound through soft tissue
is 1540 m/s (Radiological Society of North America, 2011). The speed of
sound can be found for different soft body tissues using:
Figure 3: Sound Wave
(Ultrasound for Regional Anethesia, 2008)
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V=√CρWhere: v is the speed of sound.
C is the elastic constant
p is the material density.
Due to the elastic and inertial properties of the various soft body tissue,
the speed of ultrasound travels slowest through mediums such as air and
the lung and fastest through bone (see figure 5).
3.2 Piezoelectric Transducer:
A piezoelectric transducer is an instrument used for the conversion of
electrical energy into mechanical vibrations (NDT Resource Centre, 2011).
They rely on two phenomena known as electrostriction and the
piezoelectric effect. Piezoelectric ceramic is used as the active element of
all transducers and their function is to create ultrasound waves at a
frequency in excess of 20,000 Hz (Radiological Society of North America,
2011).
Figure 4: Speed of Sound Equation
Figure 5: Speed of Ultrasound Waves through different
Biological mediums
(Ultrasound for Regional Anethesia, 2008)
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3.2.1 Electrostriction
The crystals used in piezoelectric transducers are polarized (i.e. some
parts of the molecule are positively charged, while other parts of the
molecule are negatively charged) with electrodes attached to two of its
opposite faces (NDT Resource Centre, 2011) (see figure 6). Thus, when in
the presence of an electric field, the polarized molecules will align
themselves with the electric field (see figure 7). This alignment of
molecules causes the piezoelectric crystal to change its dimensions; this is
the phenomenon known as electrostriction (NDT Resource Centre, 2011)
(see figure 8).
(Genesis Ultrasound, 2010)
Figure 7: Alignment of Molecules in Electric Field
Figure 6: Piezoelectric Transducer
(NDT Resource Centre, 2011)
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(No Author, 2007)
Thus, the piezoelectric crystal changing dimensions (i.e. expanding and
contracting) due to electrostriction leads to the production of an
ultrasound wave. The repetition of this process leads to the formation of
an ultrasound beam. An ultrasound beam is generated in pulses and each
pulse consists of approximately 2 or 3 series of the same frequency
(Radiological Society of North America, 2011) (see figure 9).
(Ultrasound for Regional Anethesia, 2008)
The distance travelled per pulse is known as the “pulse length” (see figure
9). The shorter the wave pulse, the better the axial resolution of the final
ultrasound image (Radiological Society of North America, 2011).
The Pulse Repetition Frequency (PRF) is the rate of pulses emitted by the
transducer (Radiological Society of North America, 2011) (see figure 9).
Figure 9: Ultrasound Beam
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3.2.2 The Piezoelectric Effect
Some materials, when subjected to mechanical stress, are able to produce
electricity (Hyperphysics, n.d). This phenomenon is called the piezoelectric
effect. It occurs in ultrasound when the balance of charged molecules
inside the piezoelectric crystal is disturbed (Radiological Society of North
America, 2011) (see figure 10).
(Stevens Institute of Technology, 2007)
3.3 Attenuation:
The further the ultrasound wave traverses the body and as the depth of
penetration is increased, the amplitude of the original beam is attenuated.
Thus, the energy that is lost in ultrasonic testing is known as attenuation
(NDT Resource Centre, 2011). Attenuation is due to three things:
Absorption
Reflection
Scattering of the beam
The most common of the three is absorption, which counts for 80% of all
attenuation in ultrasonic testing (Radiological Society of North America,
2011). Absorption takes the acoustic energy and transfers it into heat
production (Ultrasound for Regional Anethesia, 2008).
Figure 10: Piezoelectric Effect
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The unit of measurement for attenuation is decibels per centimeter of
tissue. This figure is represented by the attenuation coefficient of the
specific tissue type (see figure 11).
(Ultrasound for Regional Anethesia, 2008)
As the attenuation coefficient is increased, the energy lost of the original
sound beam is also increased; therefore this follows a proportional
relationship. This is why bone, which has a very high attenuation
coefficient, limits beam transmission to such a high degree. The frequency
and distance of the ultrasound beam also has an effect on the total
attenuation (Ultrasound for Regional Anethesia, 2008) (see figure 12).
Figure 11: Attenuation Coefficients of various body tissues
Figure 12: Graph of the Effect that the Frequency has on
Attenuation
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By observing the above graph it can be seen that as the frequency of the
ultrasound beam is increased, so does the attenuation. Therefore, the
frequency is directly proportional to the attenuation of an ultrasound
beam (NDT Resource Centre, 2011).
Although the majority of attenuation is due to absorption, approximately
20% of this energy loss is due to reflection and scattering. The amount of
reflection is established by the variance in acoustic impedances of the two
tissues at the boundaries (Ultrasound for Regional Anethesia, 2008).
3.4 Acoustic Impedance:
Acoustic impedance is the resistance of a tissue to the passage of
ultrasound (Ultrasound for Regional Anethesia, 2008). Acoustic impedance
can be found using a simple equation (see figure 13).
z=pv
Where: z is the acoustic impedance
p is the density of the medium
v is the propagation velocity of ultrasound through the medium.
(NDT Resource Centre, 2011)
A large difference in acoustic impedance between mediums is called
“acoustic impedance mismatch”. The greater the acoustic mismatch the
greater the percentage of ultrasound reflected and less transmitted (NDT
Resource Centre, 2011).
There is a variation of acoustic impedance with different body tissues (see
figure 14).
Figure 13: Acoustic Impedance Formula
(Ultrasound for Regional Anethesia, 2008)
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(Ultrasound for Regional Anethesia, 2008)
Analysing this table shows that the strongest reflected ultrasound beams
would be for air as it has very small acoustic impedance when compared
to other body tissues. Bone would also have a high degree of reflection as
it has an acoustic impedance that is very high relative to other body
tissues (Ultrasound For Regional Anethesia, 2008).
The reflection co-efficient can be modeled and calculated by the following
equation (see figure 12).
R=(Z2−Z1Z2+Z1 )2
Where: R is the reflected wave
Z1 is the initial impedance
Z2 is the final impedance
Figure 14: Variation of Acoustic Impedance with Body Tissues
Figure 12: Reflection Co-efficient
(NDT Resource Centre, 2010)
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For example, the reflection between muscle and bone is 41%, meaning
41% of the beam is reflected when the wave travels from muscle through
to bone (see appendix 1).
The table also demonstrates why it is necessary for a gel to be applied to
the patient when an ultrasound test is in progress. The gel acts as an
“acoustic coupling medium” on the transducer and its purpose is to
eliminate any air pockets between the transducer and the skin surface.
Without this gel, 99.9% of the ultrasound beams will be reflected away,
therefore only 0.01% the beam will be available to penetrate body tissue
and produce an image (see appendix 1) (Ultrasound for Regional
Anethesia, 2008).
4.0 Comparison to alternatives:
Ultrasound imaging yields many benefits, however it also carries some
disadvantages (see figure 15).
Figure 15: Advantages and Disadvantages of Ultrasound
Advantages Disadvantages
Very easy to perform an ultrasound
test.
The resolution of images is quite
often restricted.
Examinations are non-invasive. The
body does not have to be cut open
and nothing has to be inserted
inside the body.
The reflection of sound waves when
passing from tissue to gas has an
effect that limits ultrasound from
being used in areas containing gas,
such as the lungs.
Examinations are quick, convenient
and relatively inexpensive.
Due to attenuation, ultrasound does
not pass well through bone,
therefore images of the brain can
be taken but will often be distorted.
Does not use ionizing radiation,
therefore no harmful effects can
come from these examinations.
Extremely good at examining soft
tissue such as the eye, heart and
other internal organs.
Provides real-time imaging,
therefore it can be a good tool for
procedures such as needle biopsies.
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As ultrasound testing examines such a diverse range of problems within
the body, it can be used as an alternative to most other forms of medical
imaging. Due to this fact, there is no other form of medical imaging that
uses similar technology to ultrasound. However, as an example,
ultrasound can image the brain and so can MRI, therefore the comparison
will be made between ultrasound and MRI in order to determine the most
effective method of imaging the brain.
Magnetic Resonance imaging, or MRI, is another form of non-invasive
medical imaging that is used to evaluate and diagnose problems with
internal organs (Radiological Society of North America, 2010). MRI uses a
very powerful magnetic field and radio frequency pulses in order to
generate an image of the brain (Radiological Society of North America,
Figure 16: Advantages and Disadvantages of MRI
Advantages Disadvantages
Examinations are non-invasive. Due to the powerful magnetic field,
patients with pacemakers and other
cannot undergo a MRI procedure.
Patients are not subjected to
ionizing radiation.
Cannot always distinguish between
malignant tumors and benign
diseases.
Enables the detection of problems
they may be behind bone without
any distortion of the image.
A small number of risks and side
effects are present.
Enables the detection of
abnormalities of the brain, as well
as the assessment of the normal
functional anatomy of the brain.
Patients can become claustrophobic
in the small area inside the MRI
machine.
Larger patients may not fit inside
the MRI machine.
Examinations can take up to 4
hours; during the time patients are
required to stay motionless.
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2010). Like ultrasound, MRI carries many advantages and disadvantages
(see figure 16).
By analysing figures 15 and 16 it can be seen that both ultrasound and
Magnetic Resonance Imaging yield many benefits. Both forms of imaging
do not require any ionizing radiation as ultrasound uses sound waves to
image the patients, whilst MRI uses radio waves that redirect the axis of
spinning protons in a powerful magnetic field (Radiological Society of
North America, 2010). In addition, neither forms of imaging are invasive,
however both are effective at imaging soft tissue such as the brain. Due to
this, and the fact that both forms of imaging are non-invasive to the
patient, it can be said that both ultrasound and MRI are effective forms of
brain imaging (Simmons, n.d).
However, due to the large attenuation of the ultrasound wave when
passing through bone (the skull), the final image can be distorted,
whereas magnetic resonance imaging passes through bone, leaving a
high-resolution image. It is due to this that magnetic resonance imaging is
the most effective way of non-invasively imaging the brain.
5.0 Future developments:
Ultrasound is a rapidly developing form of medical imaging and has come
a long way since it began in the late 1930’s. Originally, all ultrasound
pictures produced were two-dimensional images. The 1980’s saw the
introduction of three-dimensional images that works in the same way as
2D, however the transducer is moved in a circular motion over the organ,
tissue or fetus to be imaged and pictures are taken from multiple angles
and planes. This renders the surface of the organ or fetus and a sculpture
like image is created (Genesis Ultrasound, 2010).
Future developments in ultrasound are beginning to emerge now in the
form of four-dimensional Doppler ultrasound. Unlike 2D and 3D ultrasound,
4D ultrasound uses several pictures in rapid succession that gives the
effect of motion. This effect is achieved by the phenomenon of the Doppler
effect. The transducer detects movement inside the body by the
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frequency of the returned echoes. A high frequency that is received
means the object is moving closer to the transducer and lower frequencies
mean the object is moving away. The images received are immediately
displayed on the display screen is such quick succession that the
movements inside the body can be seen in real-time (Genesis Ultrasound,
2010).
Even more advanced from 4D imaging is hologram ultrasound. Hologram
ultrasound allows the ultrasound beam to pass completely through the
object it is imaging whilst a second wave is transmitted simultaneously in
order to create a 3D hologram (see figure 17).
(Genesis Ultrasound, 2010)
The resulting hologram images would be capable of displaying images in
great finer detail than currently possible with ultrasound and even MRI
scans. For example, currently ultrasound cannot detect calcification, which
Figure 17: Holographic Ultrasound
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is an early marker of breast cancer, however hologram ultrasound would
provide images with such fine detail as the sound waves are passing
completely through the tissue or organ, there would be no ‘clouding’ of
the image (Genesis Ultrasound, 2010). Therefore with the introduction of
hologram ultrasound in the near future, the number of patients killed by
breast cancer every year could be drastically reduced due to the early
detection that this form of ultrasound could provide. Additionally,
cardiovascular disease could also be detected much earlier using this form
of ultrasound as the image would provide clear pictures of inside various
parts of the heart, including the pulmonary veins, left and right ventricle
and the
left and
right
atriums (see figure 18).
Figure 18: Diagram of a Heart
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The detail that holographic ultrasound images produce could in fact lead
to the detection of heart-valve disease or hardening of the arteries in such
early stages that is not at the moment possible (Genesis Ultrasound,
2010). Once again, this could significantly reduce the amount of patients
who die from preventable diseases such as these cardiovascular
infections.
6.0 Conclusion:
The aim of this report was to investigate ultrasound as a form of medical
imaging. It was found that the history of ultrasound dates back to 500BC
when Pythagoras experimented with sound and its properties. It was also
found that ultrasound is a very effective way of imaging the body non-
invasively, using mechanical sound waves produced from piezoelectric
crystals expanding and contracting due to electrostriction and the
piezoelectric effect. When compared to Magnetic Resonance Imaging,
ultrasound proved to be the less efficient way of imaging the brain
because of the distortion of the ultrasound image due to the attenuation
that the bone in the skull produces. Lastly, it was found that hologram
ultrasound is a future development that would have positive effects on
diagnosing breast cancer when it comes into use in the near future.
7.0 References:
B.A.T.S Research Group. (2011). Physics of Ultrasound. Retrieved July 10,
2011, from Better Anethetics Through Sonography (BATS):
http://www.bats.ac.nz/resources/physics.php
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Genesis Ultrasound. (2010). 4D Ultrasound uses the Doppler effect to
show motion. Retrieved August 1, 2011, from Genesis Ultrasound:
http://www.genesis-ultrasound.com/4D-ultrasound.html
Genesis Ultrasound. (2010). A Condenced History of Ultrasound. Retrieved
August 1, 2011, from Genesis-Ultrasound: http://www.genesis-
ultrasound.com/history-of-ultrasound.html
Genesis Ultrasound. (2010). Hologram Ultrasound. Retrieved August 2,
2011, from Genesis Ultrasound: http://www.genesis-
ultrasound.com/Hologram-Ultrasound.html
Hyperphysics. (n.d). Piezoelectric Effect. Retrieved July 16, 2011, from
Hyperphysics:
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/piezo.html
Media College. (n.d). How Sound Waves Work. Retrieved June 26, 2011,
from Media College: http://www.mediacollege.com/audio/01/sound-
waves.html
NDT Resource Centre. (2011). Acoustic Impedance. Retrieved June 23,
2011, from NDT Resource Centre:
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultra
sonics/Physics/acousticimpedance.htm
NDT Resource Centre. (2011). Attenuation of Sound waves. Retrieved July
23, 2011, from NDT Resource Centre:
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultra
sonics/Physics/attenuation.htm
NDT Resource Centre. (2011). Basic Principles of Ultrasound Testing.
Retrieved June 23, 2011, from NDT Resource Centre:
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultra
sonics/Introduction/description.htm
23
NDT Resource Centre. (2011). Characteristics of Piezoelectric Transducers.
Retrieved June 27, 2011, from NDT Resource Centre:
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultra
sonics/EquipmentTrans/characteristicspt.htm
No Author. (2007). Introduction: The Piezoelectric Effect. Retrieved August
1, 2011, from Piezoelectric Materials:
http://www.piezomaterials.com/
Physics 247. (n.d). Physics tutorial: Ultrasound Physics. Retrieved June 29,
2011, from Physics 247: http://www.physics247.com/physics-
tutorial/ultrasound-physics.shtml
Radiological Society of North America. (2011). General Ultrasound
Imaging. Retrieved June 26, 2011, from Radiology Info:
http://www.radiologyinfo.org/en/info.cfm?pg=genus
Radiological Society of North America. (2010). MRI of the Body. Retrieved
August 2, 2011, from Radiology Info:
http://www.radiologyinfo.org/en/info.cfm?pg=bodymr
Simmons, A. (n.d). Medical Physics - Ultrasound. Retrieved June 29, 2011,
from Genesis:
http://www.genesis.net.au/~ajs/projects/medical_physics/ultrasound
/index.html
Stevens Institute of Technology. (2007). Piezoelectric Energy Harvesting.
Retrieved August 3, 2011, from 07-08 Senior Design: Group 3:
http://www.ece.stevens-tech.edu/sd/archive/07F-08S/websites/grp3/
piezoelectric.html
The Physics Classroom. (n.d). Sound Properties and Their Pereption.
Retrieved June 26, 2011, from The Physics Classroom:
http://www.physicsclassroom.com/class/sound/u11l2c.cfm
24
Ultrasound for Regional Anethesia. (2008). Basic Principles. Retrieved June
25, 2011, from Ultrasound for Regional Anethesia:
http://www.usra.ca/basic_p
8.0 Appendices:
8.1 Appendix 1:
Calculations for Reflection:
R = ?
Z1 = 1.71 (see figure 14)
Z2 = 7.8 (see figure 14)
R=(Z2−Z1Z2+Z1 )2
¿( 1.71−7.81.71+7.8 )2
¿0.41
R%=R×100
¿0.41×100
¿41%
R = ?
Z1 = 0.0004 (see figure 14)
Z2 = 1.65 (see figure 14)
R=(Z2−Z1Z2+Z1 )2
¿( 0.0004−1.650.0004+1.65 )2
¿0.999
R%=R×100
¿0.999×100
25
¿99.9%